Air-detecting device for steam sterilizers



Nov. 18, 1969 E. G. SCOFFIELD ET AL 3,479,131

AIR-DETECTING DEVICE FOR STEAM STERILIZERS Filed Nov. 4, 1966 UnitedStates Patent 3,479,131 AIR-DETECTING DEVICE FOR STEAM STERILIZERS EricGeorge Scoflield and David Christopher Love, Nottingham, England,assignors to Manlove, Alliott & Company Limited, Nottingham, EnglandFiled Nov. 4, 1966, Ser. No. 592,081 Int. Cl. A611 3/02 US. Cl. 21-103 6Claims ABSTRACT OF THE DISCLOSURE An air-detecting device for use withsteam sterilizers comprising an upwardly-extending water-cooledcondensing tube and an upwardly-extending air-collecting tube, thesebeing joined in communication with one another at their upper ends.Air-stream mixture exhausting from the sterilizing chamber, upon entryof sterilizing steam to that chamber after its prior evacuation, isadmitted to the lower end of the condensing tube. The air-collectingtube has extending horizontally from its lower end a condensatecollecting passage which has its end remote from the air-collecting tubeconnected in communication with the lower end of the condensing tube. Ifthe level of air collected within the air-collecting tube rises above, atolerable level, a temperature sensor within the device becomes bathedin this air and signals out a warning of an unsatisfactory sterilizingcycle.

DESCRIPTION OF INVENTION This invention relates to steam sterilizersand, more particularly, is concerned with equipment for ensuring thateach sterilization cycle is satisfactory in the sense that the materialsto be sterilized are maintained at an adequately high temperature for asufiicient period of time.

The general principle of operation of the control equipment on asterilizer is based on measurement of the temperature of the steam/airmixture in the sterilizer, since the-steam temperature decreases withincreasing air. contamination at any specified pressure.

When a steam sterilizer chamber is evacuated under pulsing conditions toa vacuum in the order of 20 mm. Hg the partial pressure of the residualair will be insuflicient to move the steam temperature by anysignificant amount, providing the equipment is totally air tight and noair leaks take place. If, on the other hand, there is any leakage in theequipment then the partial pressure of the air during the sterlizingstage will be increased and the sterilizing temperature will be reduced.

Where materials to be processed are present in the sterilizer chamberthey present a condensing load, and

where there is air or other non-condensing gases present.

there tends to be an increase in the concentration of these materials inthe parts of the load most inaccessible to the steam. This increase inconcentration results in incomplete steam penetration of portions of theload being at temperatures lower than those pertaining in the freechamber space. The extreme instance of this elfect is where a singlecasket loaded with standard towels is the sole load in the chamber.Under such conditions, the majority of the noncondensible vapours in thechamber become concentrated in the centre of this single casket, and ifthe partial pressure of these non-condensible gases is much in excess of0.5 mm. Hg then these gases combined with air not completely removedfrom the casket can result in a depression in temperature in the centreof the casket by several degrees. If conditions suitable for theadequate sterilization of a single casket, as described above, arepresent at the start of the sterilizing cycle, then these conditionswill also be 3,479,131 Patented Nov. 18, 1969 suitable for otherconditions of loading where the concentration of air in the load will beless acute.

A similar eifect can be observed if the quality of the steam supply tothe chamber is not satisfactory and contains non-condensible gasesresulting from an inadequate installation or unsatisfactory boileroperation.

Equally, failure to maintain temperature will result if there is afailure in the steam supply to the chamber resulting in a fall in steampressure during sterilization.

Finally, a fault in process equipment could result in inadequate removalof air from the load even if the conditions in the chamber appearadequate. Such air could make its presence felt in the vapour space inthe sterilizer chamher after commencement of sterilization as aconsequence of downward displacement.

In view of the variation in the type of load which may be placed in asterilizer, a device which may be set up to simulate the conditionspertaining to one type of load may not adequately simulate a load ofdifferent character or weight. It is the object of the present inventionto provide a device which will ensure that satisfactory predeterminedconditions are present in the sterilizer at the commencement of thesterilizing cycle and that these conditions are maintained throughoutthe sterilizing cycle whilst at the same time there is also a monitoringof the effectiveness of the process prior to sterilizing to ensure thatthis has been adequately carried out. The device presented can bearranged to:

(1) Monitor the conditions of the gases present in a sterilizer chamberat the commencement of the sterilizing process after air has beenremoved from the chamber and load, and will only permit sterilization toproceed when the non-condensible gases present are all in aconcentration below a predetermined level, and

(2) Detect failure in the steam supply during the sterilization processand interrupt the process in this event.

(3) Detect the presence of excessive quantities of air displaced fromthe load by downward displacement during sterilization and to signal afailure to sterilize in this event.

According to the present invention, an air detecting device is providedexternal to the sterilizer chamber, but connected thereto. Such a devicemay comprise a tube with access at one end to the sterilizer chamber andarranged for accumulating air or non-condensible gases separated fromthe steam as the result of condensation, with provision made for thedisposal of the steam condensate without loss of the separated air ornon-condensible gases, and for the detection of more than apre-determined volume of accumulated air by an appropriate temperaturesensing device which normally registers steam temperature but willregister a lower temperature When exposed to air.

In one arrangement, the tube is inclined and connected at its lower endto the sterilizer chamber and the temperature sensing element isdisposed at its other end, whilst a branch tube of smaller bore which issealed by condensible gases which become separated from the steam. Avariation in the angle of inclination to the horizontal will vary thevolume of non-condensible gases which need to be accumulated before theyinfluence the temperature sensing element.

In another and preferred arrangement, the condenser tube is of invertedU configuration. The U-tube may have a shorted leg connected at itslower end to the sterilizer drain line, and longer leg in which thetemperature sensing element is disposed, the longer leg having a drainconnection for condensate leading back to the drain line in anarrangement such that steam condensate collecting in said longer legseals the drain connection but cannot rise above a predetermined level.

Forms of air detecting device or simulator according to the inventionwill now be described by way of example and with reference to theaccompanying drawings, in which:

FIGURE 1 is a view in longitudinal section of one embodiment,

FIGURE 2 is a pictorial view of a second embodiment, and

FIGURE 3 is a view in section of the device of FIG- URE 2.

The device illustrated in FIGURE 1 comprises an upwardly slopingblanked-off tube 11 communicating with pipework connected to thesterilizer chamber and having a thermocouple probe 12 extending into itthrough a gland 13 at the blanked-off end. The probe is insulated fromthe metal of the tube. Surrounding a region of the tube a short distancefrom the blanked end and the probe 12 is a small jacket 14 through whichcan be circulated cold water so as to provide a simple condensor. Thetube 11 is mounted as close to the sterilizer chamber as is possible,preferably at a level below the bottom plate of the sterilizer chamberand inclined at an angle of just above horizontal to the vertical, sothat there will be ready drainage of the condensate out of the tube awayfrom the probe 12 and free circulation in the tube of the steam/airmixture in the sterilizer pipework. Just below the thermocouple probe 12the tube 11 has a depending side branch 15 sealed thereto by a gland 16and having at its lower end a horizontal run returning to the lower endof the larger tube 11.

In operation, the tube 11 is sufiiciently closely associated with thesterilizer chamber to partake of the atmosphere therein. Upon admissionof steam to the evacuated sterilizer chamber, containing a residualmixture of air and steam under reduced pressure, the residual mixturehas access to the load to be sterilized and also to the tube 11 of theair detection device. If the condensation in the tube 11 is greater thanthat in the sterilizer load and the resistance to flow between thechamber and the air detection device is less than the resistance tovapour flow from the chamber vapour space to the inside of the load,there is accumulated in the tube of the air detection device in theearly stages of the sterilizing cycle suflicient non-condensible gasesto influence the equilibrium temperature to the extent necessary toindicate the presence of these non-condensible gases.

By careful selection of the capacity of the condenser 14 and the volumein the tube 11 in which the non-coridensible gases accumulate, it ispossible to simulate quite closely the conditions within the sterilizerload and measure temperatures which are in close accord with thoseobtaining in the centre of a pack, i.e. low temperature when air ispresent and higher temperatures satisfactory for sterilizing when therehas been pruging of the air prior to sterilization.

The side branch 15 is arranged to be sealed along the lower run by steamcondensate resulting from the effect of the condensing jacket 14. Itsangle to the horizontal can be varied and consequently the volume ofnon-condensible gases, accumulated in the vertical stem 17 above thelevel of condensate 18, can be varied and therefore provides a means ofregulating the sensitivity of the device in relation to its tolerance ofnon-condensible gases i.e. with little volume this reservoir soonbecomes filled with cool non-condensible gases and these flow out on tothe thermocouple probe much earlier than they would do if there was alarge volume. Consequently, a low volume makes the equipment sensitiveto the presence of small quantities of non-condensible gases in thesteam, whilst a large pocket reduces this sensitivity.

Further, when the thermo-couple signal is employed as a control signalin an automatic or semi-automatic sterilizing cycle, it is necessarythat a very positive impulse be given to the control equipment in theevent of their being a more than acceptable quantity of non-condensiblegases present. If there was merely an accumulation of air, in the maincondenser tube 11, there would be a slow reduction of temperature andthis would reduce the sensitivity of the equipment. However, with theaddiional pocket that is provided in the branch 15, the air in thispocket is tending to cool by virtue of heat loss to the surroundingatmosphere and its own low thermal conductivity, and therfore at thestage when this reservoir becomes filled and overflows, there is acomparatively sudden marked reduction in the temperature recorded by thethermo-couple probe.

In automatic operation, the condenser can be interlocked with the dryingstage of sterilizer operation, in which case the temperature of theequipment is stabilized with cold water once per sterilizing cycle atthe end of each cycle. -Alternatively, it can be interlocked with theexhausting system so that water is circulated through the condenserduring each vacuum pulse stage and at the vacuum stage immediately priorto sterilization; or if desired water can be left on the condenser allthe time.

Modifications of the arrangement described are possible withoutdeparting from the scope of the invention. Thus, the water-cooledcondenser could be replaced by air cooling, the air coming from suchsource as a circulating fan or motor; however the degree of control overthe process in this way will not be as effective. Also the thermo-coupleprobe cOuld be replaced by some other form of temperature-sensingelement.

An alternative embodiment of the invention is illustrated in FIGURES 2and 3 of the drawings.

It comprises principally an inverted U tube 18 having a short leg 19 anda longer leg 20. The lower end of the short leg 19 communicates With thedrain line 17 of the steam sterilizing autoclave. The longer leg 20extends down below the level of the drain line 17 and its lower endportion contains a temperature sensing element 22 that projects upthrough a fluid-tight fitting at the bottom of the leg. The temperaturesensing element may be 'a thermocouple, a resistant bulb or a liquidexpansion bulb; it is adjustable in height with respect to the U tubeleg 20. The bottom end of the longer leg 20 of the U tube is com nectedback to the underside of the drain line 17 by means of a pipe 21 thatcommunicates with the drain line in the same region as the short leg 19of the U tube.

In operation, steam and entrained air enters the U tube 18 and thereturn pipe 21 quickly becomes sealed with condensate with the resultthat air is trapped above the condensate in the vicinity of thetemperature sensing element 22. The constant level of condensate ismaintained in the longer leg 20 of the U tube by virtue of the returnpipe 21 to the main drain line 17. As the air builds up above thecondensate, so it becomes cooled by contact with the condensate which isitself losing heat to the surrounding atmosphere. When sufiicient airhas accumulated to fill the space between the surface of the condensateand the sensitive portion of the temperature sensing element 22 thelatter commences to measure the temperature of the air rather than thetemperature of the mixture of steam and entrained air. The sensitivityof the arrangement and its tolerance to air entrained in the steam inthe sterilizing chamber and drain line can be adjusted by raising orlowering the temperature sensing element so as to change the distance ofthe sensitive portion above the free condensate level.

As the partial pressure of air in the sterilizer chamber is low, aconsiderably greater concentration of air in steam is necessary if thereis to be a significant temperature reduction for the purpose of creatinga signal to operate the control equipment. The salient feature of thedevice described is its ability to concentrate air in the steam withoutitself being influenced by condensate. Additional cooling of the shortleg 19 and the U tube 18 by means of a water jacket or cooling coil,such as the coil 23, increases the volume of steam condensed andconsequently the volume of air accumulating to depress the equilibriumtemperature.

As the temperature sensing element 22 normally senses the temperature ofthe steam, any lowering of steam pressure during sterilization willresult in reduced steam temperature and this also will be sensed andused as feedback to the controller. By virtue of the large capacity ofthe condensing coil provided, vapours will be continuously drawn intothe device giving ability to sample the vapours during the course of thesterilizing process and to detect air in pre-determined amount displacedfrom the load by steam.

What we claim is:

1. A device for detecting an excess of air and other non-condensinggases in the air-steam mixture exhausting from a steam sterilizerchamber, including a tube assembly comprising an upwardly-extendingcondensing tube leg provided with indirect cooling means having aninlet/ outlet connection at its lower end through which it receives saidair-steam mixture from an exhaust line of said steam sterilizer, anupwardly-extending air-collecting tube leg communicating at its upperend with the upper end of said condensing tube leg, a condensatecollection passage extending horizontally from the lower end of saidaircollecting tube leg, the end of the condensate collection passageremote from the air-collecting tube leg being in communication with thelower end of said condensing tube leg and with said inlet/outletconnection, and a temperature sensor disposed within said tube assemblyat a level above the lower end of said air-collecting tube leg and at aposition where it becomes bathed in air collected in said air-collectingtube when the level of that air has risen to a predetermined level.

2. A device according to claim 1, wherein said indirect cooling meansutilizes water as the cooling medium.

3. A device according to claim 1, wherein said indirect cooling meansutilizes air as the cooling medium.

4. A device according to claim 1, wherein the condensing tube leg isinclined relative to the horizontal, the aircollecting tube leg issubstantially vertical, and the temperature sensor is disposed withinthe upper end of the condensing tube leg.

5. A device according to claim 1, wherein the tube assembly comprises aninverted U-tube, one vertical leg of which constitutes the condensingtube leg while the other vertical leg constitutes the air-collectingtube leg and is longer than the condensing tube leg.

6. A device according to claim 5, wherein the temperature sensor ismounted within the air-collecting tube leg and is adjustable as toheight therein.

References Cited UNITED STATES PATENTS 2,112,639 3/1938 Underwood 21983,402,991 9/1968 Henfrey 21-94 MORRIS O. WOLK, Primary Examiner BARRY S.RICHMAN, Assistant Examiner US. Cl. X.R.

